The True Stellar Obliquity of a Sub-Saturn Planet from the Tierras Observatory and KPF

We measure the true obliquity of TOI-2364, a K dwarf with a sub-Saturn-mass ($M_p=0.18M_J$) transiting planet on the upper edge of the hot Neptune desert. We used new Rossiter-McLaughlin observations gathered with the Keck Planet Finder to measure the sky-projected obliquity $\lambda =7$${}^{\circ }$${}_{-{11}^{\circ }}^{+{10}^{\circ }}$. Combined with a stellar rotation period of $23.47\pm 0.29$ days measured with photometry from the Tierras Observatory, this yields a stellar inclination of ${90}^{\circ }\pm {13}^{\circ }$ and a true obliquity $\psi =15.6$${}^{\circ }$${}_{-{7.3}^{\circ }}^{+{7.7}^{\circ }}$, indicating that the planet’s orbit is well aligned with the rotation axis of its host star. The determination of $\psi$ is important for investigating a potential bimodality in the orbits of short-period sub-Saturns around cool stars, which tend to be either aligned with or perpendicular to their host stars’ spin axes.